This invention is directed to a novel process for recovering substantially pure 2,3,5,6-tetrachloropyridine from mixtures of chlorinated pyridines. Highly chlorinated pyridine compounds, such as 2,3,5,6-tetrachloropyridine, are known to be useful as pesticides or as intermediates for the preparation of other compounds having pesticidal properties. The various processes used to prepare chlorinated pyridines and especially 2,3,5,6-tetrachloropyridine have generally resulted in producing product mixes containing many chlorinated compounds including 2,3,4,5- and 2,3,4,6-tetrachloropyridine, the two position isomers of the desired 2,3,5,6-tetrachloropyridine. It has been particularly difficult to separate 2,3,5,6-tetrachloropyridine from its two position isomers by conventional economical techniques such as distillation since the boiling points of these three materials are so close. It has therefore been necessary to resort to more costly, difficult and time-consuming procedures of selective crystallization and multistage and/or multiplate distillation.
It has been discovered that both 2,3,4,5- and 2,3,4,6-tetrachloropyridine are converted to pentachloropyridine at a much more rapid rate than 2,3,5,6-tetrachloropyridine when they are chlorinated in the presence of a Lewis acid catalyst. Because of this difference in reaction rates, both 2,3,4,5- and 2,3,4,6-tetrachloropyridine can be substantially converted to pentachloropyridine prior to substantial conversion of any 2,3,5,6-tetrachloropyridine which might be in admixture with the above isomers. The separation of 2,3,5,6-tetrachloropyridine from pentachloropyridine is a rather easy procedure.